This invention relates generally to a pulse coding system, and more particularly to a multi-pulse commutator for a radar transponder.
Radar transponders adapted to be carried on aircraft or other vehicles respond to pulses received from radar transmitters and provide reply pulses which are received by the receiver at the originating radar. It is desired to code the reply pulses so that the signal received at the originating radar identifies the particular aircraft or vehicle from which the reply is received or conveys other information. In some applications a simple two-pulse reply is provided, with the spacing between the two pulses identifying the different transponders. That is, the time interval between the pulses is different for each transponder.
It is desired that the transponder equipment be small so that it will require a minimum of space on the aircraft, and also that it consumes as little power as possible. In order to meet the space and power requirements, equipment has been provided using semiconductors to the largest extent possible. To provide the radio frequency output power required, a magnetron may be used as the transmitter oscillator, and this may be effectively modulated by a line pulser type modulator that uses a silicon controlled rectifier as a switch. However, silicon controlled rectifiers require a finite time for recovery and it is, therefore, not possible to provide two closely-spaced pulses from a single, silicon controlled rectifier type modulator. Although it is possible to use a plurality of modulators, this increases the cost and size, as well as the power consumption.
The prior art multi-pulse modulator for radar transponders is disclosed in U.S. Patent No. 3,383,680, issued May 14, 1968, and assigned to the same assignee as the present invention. In this prior art modulator, the various delays are obtained through the use of a plurality of paths, each incorporating a different predetermined amount of delay and periodically gated into the circuit through the use of a plurality of gates and a switching device. While this modulator operates extremely well and greatly reduces the cost from other prior art devices, the present invention has been designed to further reduce the cost and size and to incorporate the delay circuitry into an IC, or integrated circuit.